The movie's basically systematically torturing of random people in order to get a twisted insight into the world after death of sorts. How much does it take for ones psyche to break? This is basically the main issue of the Martyrs. The film isn't a quasi-horror (much like many horror movies these days are), but it's not at all a classic horror movie either.

However, that movie's also about disguised thoughts of how far some are willing to go, to achieve certain bizarre goals, with the main mechanism being it's only a matter of how much of a fat wallet one has.

The only French horror movie that really creeped me out was À l'intérieur.